Autonomous landing of a tethered multi-rotor unmanned aerial vehicle on a stationary platform
Tatenda Joshua Mfiri · SUNScholar (Stellenbosch University) · 2019
ENGLISH ABSTRACT: This thesis contributes to the research of persistent tethered aerial vehicles by presenting a strategy to autonomously land a tethered multi-rotor UAV. A flight control system is developed for a quad-rotor UAV which was inherited from a previous project. A winch system is also added and a control system is designed to winch the tethered quad-rotor. A novel point-mass model of the tethered UAV is formulated and utilised to design a tethered landing strategy which is robust against inaccuracies in aircraft position measurement. The work culminates in the demonstration of autonomous landings of a tethered quad-rotor UAV on a 2.4 m × 2.4 m stationary platform. The flight control system is designed by using successive loops of Proportional- Integral-Derivative (PID) controllers. Flight control loops are designed based on an untethered quad-rotor model and integral control laws are implemented to aid the rejection of tether disturbances. A spring-damper model proved to be sufficient in modelling tether dynamics. A novel point-mass model of the tethered UAV is utilised in deriving closed-form analytical expressions for the tethered system poles both in the radial and angular directions as functions of quad-rotor and tether parameters. Flight control systems are designed and simulated in a software-in-the-loop environment. Control loops that are critical to the landing strategy are also simulated in a hardware-in-the-loop environment. Practical results show satisfactory performance in horizontal control in wind speeds of up to 2 m.s−1 while the vertical control system exhibits relative immunity to an increase in wind speed.